Evolution of the seed

Adaptations of seed plants Seeds Reduced gametophyte stage Also seen in seedless vascular seed plants reduced to microscopic (spores develop in sporangia of sporophyte) Heterospory Megaspores – make female gametophyte Microsporangia – male gametophyte Ovules = megaspore & integuments (protect) Pollen = micro (male gametophyte) & sporopollenin

Microsporangium: on anther

Megasporangium Ovary produces the ovule Ovule parts: INTEGUMENT MACROSPORAGIA FEMALE GAMETOPHYTE (egg)

Seed Seed : Ovule develops into seed Cotyledons: 1 or 2 seed leaves Embryo: immature plant Endosperm:food supply Seed coat of integument Ovule develops into seed Cotyledons: 1 or 2 seed leaves

Seed parts Embryo Endosperm Seed coat

Seed Double Fertilization Endosperm The union of two sperm cells with different nuclei of the embryo sac. Endosperm Food storing tissue of the seed. http://www.learner.org/channel/courses/essential/life/images/show4.open_seed.jpg

Seed to Seedling Imbibition is that state when the seed wakes up from dormancy and starts the intake of water. It causes the seed to expand and ruptures, releasing the shoot, the cotyledons and the stalk. It is the first sign of life after the dormant state.

Seed to Seedling http://students. usm. maine. edu/deidre

Spore survival Prior to seeds: spore was the only protected stage for plants. Spore is single cell Could survive: cold hot dry Could be dispersed to another environment

Spore Release

Then, seeds Seeds are multicellular Has protective integument coat from ovule Can be dormant for days months years

Seed Release

Vascular Seed plants Gymnosperms: fir, pine Angiosperm: flowering plants

Pine tree The pine tree is the sporophyte (2N) Its sporangia are located on the pine cone Heterosporous Small pollen cones: carried by wind and critters Large ovule cones Each tree has both types of cones

Female - Male female male

Angiosperm Flowering plants: vascular and seeds Most diverse and widespread plants Flower: specialized for reproduction Carried by wind: like gymnosperm Carried by insects: more directed pollination Fruit: protect seed and aid in dispersal From ovary. Ovary thickens after pollination.

Flower Structure Four main organs: sepals, petals, stamens, and carpels Stamens and carpels are reproductive Carpel: Stigma: collects pollen Style: to ovary with 1 or more ovules ovary Pistil: multiple fused carpels Stamen: filament, anther Anther: stalk where pollen made Sepals and petals: no repro organs http://images.google.com/imgres?imgurl=http://andromeda.cavehill.uwi.edu/

Gametophyte Development and Pollination Sporangia- structure on anther and ovules where spores are produced Pollen grains are made up of mature male gametophytes that are enclosed by a spore wall Found in the microsporangia, or pollen sacs

Hormones Auxin Stem elongation, root growth, develop fruit Cytokinins: affect, cell division, cell differentiation, and apical dominance. Gibberellins Seed and bud germination Abscisic acid Inhibit root growth, close stomata Ethylene Fruit ripening

Mechanisms that Prevent Self-Fertilization Sexual reproduction ensures that there will be genetic diversity among offspring “Selfing” refers to self-fertilization in plants Ensures that seed will develop

Pollination Vs. Fertilization Pollination is the transfer of pollen from anther to stigma (of other plant) Results in formation of a pollen tube Purpose is to grow down to ovary and release sperm within embryo sac This fertilizes the egg Embryo -> seed -> fruit containing seed The fruit disperses seeds which germinate and develop into seedlings

Development Microsporocytes form four haploid microspores These develop into haploid male gametophytes The microspore goes through mitosis and cytokinesis This results in a generative cell and tube cell which make up the pollen grain

Megasporocyte grows resulting in four haploid megaspores May form a multicellular female gametophyte Contains 3 cells: 1 egg and 2 synegrids Synegrids attract and guide pollen tubes to the embryo sac there are also two nuclei at the other end of the cell that are not separate and share cytoplasm embryo sacs These result in two integuments that form a seed

Ensuring that the egg and sperm cells come from different parents is a mechanism that inhibits self fertilization Dioecious plants cannot self fertilize because they have either staminate or carpellate flowers

Avoid self fertilization Flowers with functional stamen and carpels have organs that mature at different rates An animal pollinator would not transfer pollen from the anther to a stigma of the same flower

Self Incompatibility The ability of a plant to reject its own pollen Also, in some cases, the pollen of similar plants If pollen were to land on the stigma of a flower on the same plant a biochemical would prevent the pollen from developing and fertilizing an egg

Special Circumstances Plant breeders will hybridize different crop varieties to combine the best traits and to get sufficient results This can result in inbreeding May cause plants that are able to self fertilize to lose that characteristic.